Wet process phosphoric acid is produced by the acidulation of phosphate rock with sulfuric acid. Calcium sulfate precipitates in this reaction, usually as gypsum, and is separated by filtration or other suitable means from the resulting phosphoric acid which is usually about 30% P.sub.2 O.sub.5 in strength. The acid may next be concentrated by evaporation or any other suitable means to a 54-60% P.sub.2 O.sub.5 merchant grade product or, sometimes, to a lower strength for use, for example, in the manufacture of a fertilizer such as diammonium phosphate. Phosphoric acid in medium strength is also frequently found at various intermediate stages in the manufacture of the acid by the wet process.
The phosphate rock which is acidulated almost always contains numerous natural impurities which dissolve in the sulfuric acid or which otherwise pass through the gypsum filters with the phosphoric acid as suspended fines. Among these nautral impurities are siliceous materials such as SiO.sub.2, iron and aluminum compounds such as clays, alkali metal compounds, magnesium compounds, fluorides, uranium and rare earths, as well as some carbonaceous matter such as humic materials and highly condensed organics of marine origin. In addition to these natural impurities, the weak 30% phosphoric acid separated from the gypsum also often contains impurities which originate from organic substances such as fatty acids, fatty acid derivatives, tallow amines and hydrocarbon oils that are added during the beneficiation and acidulation of the phosphate rock. Finally, excess sulfuric acid is normally used in acidulating the rock and this leads to the presence of substantial amounts of sulfates in the 30% phosphoric acid.
While the impurities in wet process phosphoric acid are generally regarded as contaminants which detract from quality and appearance and often interfere with the subsequent processing of the acid, some are considered potential sources of by-product recovery. Various solvent extraction processes have therefore been developed for extracting the impurities or potential by-products from phosphoric acid or, in some cases, for extracting the phosphoric acid from the impurities or potential by-products. Examples of such solvent extraction processes are described in U.S. Pat. Nos. 3,737,513 and 3,711,591 (recovery of by-product uranium), No. 3,700,413 and No. 3,437,454 (recovery of by-product vanadium), No. 3,458,282 (removal of fluorine or anionic sulfate), and No. 3,694,153 (removal of iron and alkaline earth metals).
In the typical solvent extraction process, a water insoluble organic extractant phase containing an active complexing agent is mixed with the phosphoric acid. The impurities or potential by-products are transferred to (extracted into) the extractant phase and the pregnant extractant is separated from the acid in a settler or other suitable equipment. Alternatively, the phosphoric acid is transferred to (extracted into) the extractant phase and the acid-pregnant extractant is separated from the raffinate containing the impurities or by-products, again in a settler or other suitable equipment. Usually, multiple countercurrent mixer-settler stages are employed for this operation, and the loaded organic extractant is subsequently treated in some manner to separate the extractant component and regenerate a "lean organic" for reuse in extraction of the acid. The organic extractant phase may include liquid hydrocarbons, similar to those employed in the present invention, which are used as diluents, or carriers, of the complexing agents used as extracting ingredients, but it will be understood that the present invention differs from these solvent extraction processes in that the present invention does not require an active complexing agent and in that the present invention involves physical removal of substances which are visibly distinguishable from, and substantially insoluble in, the hydrocarbons with which they are removed; whereas the solvent extraction processes require an active complexing agent and involve the extraction, or dissolution, of substances which are then dissolved in the solvent and are not visibly distinguishable from the solvent with which they are extracted.
It has been recognized that phosphoric acid should be subjected to a clarification operation prior to extraction as described above. Among the reasons for this are that a high concentration of solids in the phosphoric acid feed tends to decrease the operating efficiency of the equipment which is used to carry out the extraction and that the presence of solids in increased quantities leads to increased extractant losses, since the extractants tend to adhere to the solid particles and leave with the aqueous phase, thereby becoming unavailable for extraction. In addition, the presence of solids in the acid has generally been associated with the formation of an interfacial layer (generally referred to by those skilled in the art as "crud") during extraction, which in turn results in a more difficult separation of phases during the extraction operation, which can clog or otherwise interfere with the operation of the extraction equipment, and which may deposit on the surface of the extraction equipment, adding to the problems of cleaning, maintenance, etc. It has been recognized that to some extent the degree of crud formation is related to the degree of clarification of the acid prior to extraction and that well-clarified acids normally produce less crud during extraction than acids which have high solids contents. However, even with a well-clarified phosphoric acid, crud formation still occurs along the extractant-acid interface and this creates the above-mentioned problems of separation of phases, extractant losses, and removal and disposal of the crud.
The formation and actual composition of the material referred to herein as crud is not completely known. The material appears to be made up mainly of solid compounds which come into the system as impurities with the phosphate rock or as additives during beneficiation and acidulation of the rock. These compounds are generally referred to by those skilled in the art and in this disclosure as crud-forming agents. They include:
1. Siliceous materials, such as SiO.sub.2, Na.sub.2 SiF.sub.6, and K.sub.2 SiF.sub.6.
2. Iron and aluminum compounds, such as AlF.sub.3, and a number of clays, such as (Mg.Ca)O.Al.sub.2 O.sub.3.4SiO.sub.2.nH.sub.2 O (where n=2, 3, 4, etc.) and Al.sub.2 O.sub.3.2SiO.sub.2.2H.sub.2 O.
3. Sulfates, from the reaction of the H.sub.2 SO.sub.4 with various cations, such as CaSO.sub.4, CaSO.sub.4.2H.sub.2 O, and MgSO.sub.4.
4. Fine carbonaceous solids, usually black, which apparently come from the phosphate rock, and which are virtually insoluble in liquid hydrocarbons. These are usually found almost completely incorporated into the crud. Examples are humic materials, such as humic acid which is a material often found in dirt; highly condensed organics of marine origin, which are similar to the humic materials; and the like.
5. Fatty acids, which may have been introduced during beneficiation. Fatty acids of the type used in beneficiation have the general formula R--COOH, where R is a high molecular weight alkyl radical usually having at least about 10 carbons. Examples include palmitic acid, CH.sub.3 (CH.sub.2).sub.14 COOH and stearic acid, CH.sub.3 (CH.sub.2).sub.16 COOH.
6. Tallow amines, which may have been introduced during beneficiation. Tallow amines are primary amines derived from tallow fatty acids. They are high molecular weight aliphatic amines produced from mixtures of fatty acids. Normally they are mixtures of amines of the type represented as R--CH.sub.2 --NH.sub.2, where R is a radical of more than 10 carbons and the total number of carbons is usually even.
7. Fatty acid derivatives, which may come from the use of defoamers during acidulation. Fatty acid derivatives used in defoamers usually include esters of the type represented as R--COO--R.sub.1, where both R and R.sub.1 are high molecular weight alkyl radicals, each having at least about 10 carbons. Examples are waxes such as C.sub.23 H.sub.49 COOC.sub.26 H.sub.53 and C.sub.27 H.sub.55 COOC.sub.26 H.sub.53.
8. Any other organic and inorganic compounds present in the phosphate rock, such as fluorides (e.g., MgF.sub.2, CaF.sub.2, and NaF), alkali earth metal compounds (e.g., Na.sub.2 SiF.sub.6 and KF), and alkaline earth metal compounds (e.g., MgF.sub.2 and CaF.sub.2). The proportion of each of these crud-forming agents varies from acid to acid, depending on such factors as the source and nature of the phosphate rock used to make the acid. In some acids the the siliceous (SiO.sub.2 -like) materials may comprise as much as 80% or more of the crud-forming agents, while in other acids the aluminum and iron compounds may constitute most of it. In still other acids, the carbonaceous matter may make up as much as 50 or 60 percent of the crud-forming agents. Whatever the make-up and composition of the crud-forming agents, they are all substantially insoluble in the liquid hydrocarbons used in accordance with this invention. They also have the common property that they are not dissolved, or extracted, by the solvent extractants used in solvent extraction processes. Rather, they tend to accumulate in the interface during solvent extraction and cause the problems and inconveniences mentioned above. The materials which form the crud appear to do so by encapsulating droplets of extractant or otherwise thickening the extractant. Typically, the extractant becomes a thick mass when contaminated with as little as 1% of the crud-forming materials, that is, one pound of these materials may tie up about 100 pounds, or more, of extractant, which then become unavailable for extraction of the acid.
In some cases, the amount of crud formed appears to be related to the strength of the acid being processed. Thus at concentrations below about 40% P.sub.2 O.sub.5, phosphoric acid can be clarified to very low solids contents by settling. A high molecular weight anionic flocculant is often used to aid in the removal of the fines by settling techniques. Low strength acids, e.g., 30% P.sub.2 O.sub.5, which have settled for several days generally do not form excessive amounts of crud. On the other hand, high strength acids, e.g., 60% P.sub.2 O.sub.5, are usually more difficult to settle and generally form more crud. In any event, it is often not economically feasible to provide several days of settling in commercial installations, nor is it always desirable to carry out the solvent extraction at low acid strengths. Thus the problems of maintenance of extraction equipment, removal, handling and disposal of crud, expenses associated with crud formation extractant losses, and the problems of producing ultraclear acid feed stocks are often of sufficient magnitude to make any of the proposed phosphoric acid solvent extraction processes commercially unattractive.
It is therefore an object of this invention to avoid or substantially reduce the formation of crud during solvent extraction of wet process phosphoric acid.
It is another object of this invention to provide a method for treating wet process phosphoric acid to substantially remove crud-forming agents from the acid prior to solvent extraction of the acid.
It is yet another object of this invention to provide a method for treating wet process phosphoric acid so that no crud, or at least substantially less crud, forms when the acid is subsequently subjected to solvent extraction.